1. Field of the Invention
The present invention relates to a method for producing a directed aerosol stream for coating substrates, and in particular to a method wherein an aerosol stream is produced from a gas and/or vapor mixture by way of a flame-free chemical reaction and is surrounded by a moving, essentially aerosol-free gas and/or vapor stream to form a directed aerosol stream enveloped in the aerosol-free stream.
2. Discussion of the Art
Aerosols are supsensions of solid or liquid particles in a gas, the particles often being in the colloidal size range. Aerosols containing solid particles are referred to as soots or fumes and aerosols containing gases or vapors as well as liquid particles are called fogs. An exemplary process for producing a flowing aerosol, hereinafter referred to as an aerosol stream, provides that gaseous and/or vaporous chemical components are mixed with the aid of a diffusion process and/or a turbulent mixing process and the resulting gas and/or vapor mixture is converted by thermal reaction, e.g. flame hydrolysis, into an aerosol stream.
In conventional aerosol streams, the solid or liquid particles move at various angles and in various directions. An aerosol stream of this sort is often uneconomical to use when a directed stream is needed, e.g., in manufacturing facilities which do coating, where it is desirable that only the articles to be coated are covered and not the surroundings. Such a conventional aerosol stream may be directed by mechanical means, e.g., using baffles. However, this procedure is uneconomical since the baffles are also coated by the aerosol particles, which result in losses of the aerosol and requires expensive cleaning procedures.
In copending, commonly assigned, U.S. patent application Ser. No. 632,291 by Reinhard Kuhne and Max Kuisl, filed July 19, 1984, the disclosure of which is herein incorporated by reference, a method for producing a directed aerosol stream is provided which results in an economically manageable aerosol stream having a high particle density and which is suitable, in particular, for coating systems.
The method is particularly suited for the production of so-called preforms for optical fibers made of quartz glass. For this purpose, gaseous silicon tetrachloride (SiCl.sub.4) is reacted in a flame-free chemical reaction in a reactor with water vapor (H.sub.2 O), preferably at 500.degree. C. to 1000.degree. C., so as to produce an aerosol composition containing silicon dioxde (SiO.sub.2) particles. A directed aerosol stream is produced by causing the aerosol composition to flow as an aerosol stream and enveloping the aerosol stream in an essentially aerosol-free gas and/or vapor stream by which it is conducted and directed. A rod-shaped substrate is disposed in the directed aerosol stream and the SiO.sub.2 particles are coated or precipitated, in the form of one or more layers, on this substrate.
Optical preforms may require layers having different optical indices of refraction to thereby provide a refractive index gradient or profile. The index of refraction may be increased by adding hydrolyzable substances, e.g., germanium tetrachloride (GeCl.sub.4), to the gaseous silicon tetrachloride (SiCl.sub.4) in the reactor which result in the formation of doped, e.g., germanium doped, silicon dioxide particles.
In the prior art method, SiO.sub.2 particles are produced by a hydrolysis reaction accroding to the following formula: EQU SiCl.sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +4HCl (1)
Doping of SiO.sub.2 particles produced in this manner is taught as possible only with doping substances which can be incorporated in the quartz matrix under the above-mentioned reaction conditions. Such a hydroloyzable substance is, for example, germanium chloride (GeCl.sub.2), which results in germanium doping of the silicon dioxide particles produced and increases the optical index of refraction thereof. This doping takes place with the aid of a hydrolysis reaction which takes place analogously to Formula (1) and according to the following formula: EQU GeCl.sub.4 +2H.sub.2 O.fwdarw.GeO.sub.2 +4HCl (2)
However, in order to fabricate an optical fiber having an optical index of refraction gradient or profile which varies broadly, it is desirable that the optical index of refraction of the SiO.sub.2 be reduced by doping with a substance which may be introduced into the SiO.sub.2 particles in the aforementioned prior art flame-free process.